1. Field of the Invention
This invention relates to tools used in the testing of subterranean wells, and concerns in particular the mechanism by which such tools--especially but not exclusively those for use in hydrocarbon-bearing wells--are operated.
2. Description of The Prior Art
Whether at sea or on land, the first stages in the production of a new hydrocarbon well--an oil well--are the drilling of the well bore itself through the various formations within the earth's crust beneath the drilling rig, followed by "casing" (the introduction and cementing into position of piping which will serve to support and line the bore) and the introduction into the bore, at the depth of a formation of interest, of a device known as a packer, into which inner tubing (of smaller diameter than the casing) can subsequently be lodged.
The next work carried out is normally some programme of testing, for the purpose of evaluating the production potential of the chosen formation. The testing procedure usually involves the measurement of downhole temperatures and pressures, in both static and flow conditions (the latter being when fluid from the relevant formation is allowed to flow into and up the well), and the subsequent calculation of various well parameters. To collect the necessary data there is used a test string--a length of tubing containing the tools required for the testing--that is lowered into the well bore to the required (test) depth. Either the packer has previously been placed at that depth, and the test string is then set into the packer, or the packer is sent down as part of the test string, and then set into place in the bore; in any event, once the string is set in the packer and the packer is set in the bore, the tubing of the string is isolated from the surrounding well.
One essential component of the test string is a valve known as the downhole valve, which is used to control the flow of fluid out of the formation and into and up the well tubing. The density of drilling fluid in the tubing above this valve is adjusted such that its hydrostatic pressure at the depth of the formation is lower than the formation fluid pressure. Thus, when the valve is opened, formation fluid is permitted to enter the well bore through perforations in the casing and flow into the tubing string (and possibly to the surface therethrough). This contrasts with the situation during drilling, when the drilling mud must exert a hydrostatic pressure greater than the formation fluid pressure in order to prevent the formation fluid's escape to the surface.
The operation of the various tools included in the downhole test string, including the opening and closing of the downhole valve itself--and, consequently, the control of the testing procedure--can be effected using one of three main types of mechanism. These types are those actuated by reciprocal motion of the pipe string (the inner tube, of which the test string constitutes a part), by rotational motion of the pipe string, or by changes in the pressure differential between the tubing and the annular space which surrounds it in the well--hereinafter referred to simply as "the annulus". Test strings wherein the tools thereof are activated by changes in annulus pressure are at present much in vogue, and it is this type of mechanism with which the invention is particularly concerned.
A mechanism of the annulus pressure-responsive type requires the provision and maintenance of a fixed "reference" pressure within the tool. This, used in conjunction with an adjustable (and higher) annulus pressure, allows the establishment of the chosen pressure differential necessary to control the operation of the appropriate component of the test string.
To ensure that the downhole tools operate within a narrow known band of applied annulus pressure, it is essential that a constant reference pressure be established within the tool string. A convenient such pressure to trap is the hydrostatic ambient (annulus) pressure experienced by the string after it has been lowered down the well bore and set into the packer. This annulus pressure may, through a suitable connection, be communicated to a gas-filled pressure chamber within the string. However, once trapped the reference pressure must be isolated from both the annulus and the tubing so that fluctuations in the pressures therein will not affect the reference pressure. Allowance must also be made for the commonly-encountered situation wherein there is a pressure increase within the tubing, during stabbing into the packer, due to a "pistoning" effect (the annulus liquid being displaced by the descending tubing can no longer escape up past the tubing once the latter has reached, and is being stabbed into, the packer, so there is a pressure build-up)--this excess pressure must be dissipated, and not communicated to the reference pressure chamber.
Variations in environmental temperature tend, via thermal expansion and contraction of the pressurised gas, to alter the reference pressure, and so it is unfortunately also preferable to provide some means of compensating for this. Finally, additional temperature compensation may be required if, as is quite common, certain procedures known in the Art as stimulation, which attempt to improve the oil yield of the formation, are employed once the initial well testing is completed Two examples of such procedures are hydraulic fracturing and acid stimulation. Their details are not relevant here, except inasmuch as they may require the pumping to the formation, via the test string, of fluids that are cold relative to the formation temperature--acids, for example. A pumping operation of this kind will cause the reference pressure to drop, due to contraction of the gas as it cools, unless some provision is made to maintain it--and, furthermore, the pressure will rise again once the pumping has ceased unless once more it is adjusted. Analogous problems can similarly occur during the pumping (albeit rare) of hot fluids to the formation--for example, to help remove waxy deposits blocking the perforations in the casing.
All these situations, then, require some suitable means first of isolating and then of maintaining the reference pressure in order that it should remain constant (normally at the true hydrostatic pressure) under any foreseeable conditions, thus allowing a known pressure differential to be created between the tool and the annulus simply by raising the annulus pressure to a predetermined level.
It is these means that the invention seeks to provide. Firstly, the invention proposes that reference pressure within the test string be trapped by a novel mechanism wherein a valve drivable into a closed position by a first piston open to annulus pressure first defines, and then defines and closes, the open-to-tubing-pressure entrance to a passageway leading to a reference-gas-containing chamber via a second piston therewithin. Using this mechanism, firstly, as the open-ended test string is lowered into the wall bore, tubing pressure is in equilibrium with annulus pressure, and is communicated via the passageway entrance and the chamber-contained piston to the reference gas, and secondly, after the test string has been stabbed into the packer, so isolating tubing pressure from annulus pressure, a momentary increase in annulus pressure will cause the first piston to move to drive the valve into the passageway-closed position, thus effectively sealing off the trapped reference gas from any further pressure changes.
Secondly, the invention proposes a new mechanism by which compensation can be made for the effect of downhole temperature changes on the gas in a reference pressure chamber, in which mechanism there is a hydraulic-liquid-containing chamber which is connected at one end, via a piston thereat, to a vent to annulus and at the other end to two "one-way" passageways linking it to the reference-gas-containing chamber via a chamber-contained second piston. With this mechanism, upon cooling (and thus contraction and pressure reduction) of the reference gas the resultant excess annulus liquid pressure is communicated to, and exerted on, the second piston via the first piston and the hydraulic liquid, thus causing a movement of the second piston which will re-compress the gas and restore reference pressure. Similarly, upon heating (and expansion and pressure increase) of the reference gas, the resultant excess gas pressure is communicated to, and exerted upon, the first piston via the second piston and the hydraulic liquid, thus causing a movement of the first piston to vent chamber-contained annulus fluid, and thereby allowing movement of the second piston which will decompress the gas and restore reference pressure.